FRISCO — Valiant weed warriors, who have made it their mission to try and eradicate non-native plants, may want to think about the bigger ecological picture as they plan their weekend weed pulls.

A new study led by the University of Pittsburgh’s Susan Kalisz suggests that, in some cases, invasive plants overwhelm native ecosystems because of an overpopulation of deer. The density of deer in the United States is about four to 10 times what it was prior to European settlement of North America. That density, Kalisz posits, is the main reason garlic mustard is crowding out native plants, such as trillium, which are preferred food for wild deer.

Kalisz, a professor of evolutionary ecology in the University of Pittsburgh’s Department of Biological Sciences, said native plants can often crowd out invaders without other external factors that tilt the playing field in favor of the invaders.

The findings of the study were published this week in the Proceedings of the National Academy of Sciences.

“This demonstrates that the high population growth rate of the invader is caused by the high abundance of deer,” Kalisz said. This effect is reversible with deer exclusion, she added.

The study was initiated in 2003 at the Trillium Trail Nature Reserve in Fox Chapel, Pennsylvania to learn why garlic mustard is crowding out native plants, such as trillium, which are preferred food for wild deer.

Garlic mustard (Alliaria petiolata) is a plant native to Europe and Asia and is inedible by deer standards. It was brought to the United States—Long Island, N.Y., specifically—in the 1860s for use as a kitchen herb.

Instead it became a menace, colonizing forest floors in the Eastern United States and Canada and has been found in Washington, Utah, and British Columbia, achieving the dubious distinction of being one of very few non-native plants to successfully invade forest understories. The persistence of garlic mustard greatly reduces forest biodiversi

Kalisz and colleagues established multiple 196-square-meter plots in the forest. Half were fenced to exclude deer. Years of observation and hours of statistical analysis later, Kalisz and her colleagues have found that in plots where deer were excluded, the trillium population is increasing and the garlic mustard population is trending toward zero.

This study shows that two major ecological and management problems are linked: an excess of deer in temperate forests and an invasion of these forests by exotic plants. Similar links may be found in other ecosystems between overabundant native or managed fauna (such as cattle or sheep) and declining diversity of flora. Management of overabundant animals could be beneficial for conservation of plant biodiversity in general.

“When people walk in the woods where deer are overabundant, they don’t realize what’s missing,” Kalisz said. “They don’t know what used to be growing there. They don’t know that species are being lost and replaced by invaders.”

The solution seems simple, then: Reduce deer populations, restore natives, and prevent invasion. It’s not simple, Kalisz says. Deer management policies vary from state to state, and deer don’t respect political boundaries. Some states keep deer populations low, while others prefer to maintain higher populations to appeal to groups such as hunters. Yet, deer, Kalisz says, exact a toll not only on forest species but also on farms, orchards, and even your car and even car insurance rates.